Efficient genetic modification of animals, especially in higher mammals, has been a major goal of researchers in the biotechnology field for the last two decades. Not only can genetic modification of animals advance our understanding of genes and gene-functions in multi-cell organisms, it can also serve useful applications in the bio-agricultural industry as well as bio-drug industry. Examples of these applications include raising livestock with desired characteristics such as faster growth rate, production of therapeutic proteins in milk, or even the generation of more “humanized” organs from animals for use in animal to human xenotransplantation. Genetic modification of animals with human genes are also valuable as pharmaceutical bioreactors. Human proteins expressed in transgenic animals such as chicken also provide a better economic alternative to bacteria or yeast expression systems due to their reduced production costs.
In theory, transgenic chickens should be ideal bioreactors for making large quantities of recombinant proteins. However, due to the large size of avian eggs and the difficulty in harvesting an egg before it has begun developing into a chick has greatly hindered progress in this field. The most common way to make a transgenic animal is to harvest a newly fertilized oocyte and inject foreign DNA directly into nucleus using microinjection technology. While this task has become fairly routine in mammalian transgenics, it is not the case with avian transgenics. Not only are chicken zygotes difficult to harvest from the chicken's oviduct, but once harvested, the single cell is difficult to locate within the viscous yellow yolk.
Current techniques to modify the genome include microinjection of foreign DNA into the pronuclei of fertilized eggs, delivery of foreign DNA into embryonic stem cells in vitro or blastomere cells in vivo through lipid-based agents, electroporation, or viral infection. Aside from mice, however, genetic modification techniques have had limited success in other animals. The microinjection technique, for example, has been reported to be technically very demanding and requires the use of highly sensitive and expensive equipment. The viability of embryos after microinjection has also been reported to be very poor. Wall, R. J., et. al. (1992) Making Transgenic Livestock, Genetic Engineering on a Large Scale, Journal of Cellular Biochemistry, Vol. 49, pp. 113-120. This has led researchers in the field to investigate alternative and easier ways of delivering genes into an animal.
In 1989, Lavitrano, M., et. al. reported that simply incubating foreign DNA with mice's sperm cells and effecting fertilization in vitro could lead to genetically modified mice. Lavitrano, M., et. al. (1989) Sperm Cells as Vectors for Introducing Foreign DNA into Eggs—Genetic Transformation of Mice, Cell, Vol. 57, pp. 717-723. Characterized as the “cold fusion” equivalent in biotechnology, this report generated much excitement in the field. Birnstiel, M., et. al. (1989) Dangerous Liaisons: Spermatozoa as Natural Vectors for Foreign DNA?, Cell, Vol. 57, pp. 701-702. Those skilled in the art, however, are reported to remain skeptical even to this day about the Lavitrano's report since a number of researchers in the field have reportedly failed to repeat the experiment. Brinster, R., et. al. (1989) No Simple Solution for Making Transgenic Mice, Cell, Vol. 59, pp. 239-241; Smith, K. (1999) Sperm Cell Mediated Transgenesis: A Review, Animal Biotechnology, Vol. 10(1&2), pp. 1-13.
Over the last decade, efforts have continued to explore the use of sperm cells as a vector for mediating gene transfer in animals. Researchers have elucidated that sperm cells have the inherent ability to internalize foreign DNA. Francolini, M., et. al (1993) Evidence for Nuclear Internalization of Exogenous DNA into Mammalian Sperm Cells, Mol. Reprod. Devel., Vol. 34, pp. 133-139. Yet, certain inhibitory factors present in seminal fluid may inhibit this ability to take up DNA. Lavitrano, M., et. al. (1992) The Interaction Between Exogenous DNA and Sperm Cells, Mol. Reprod. Devel., Vol. 31, pp. 161-169. In addition, foreign DNA introduced into sperm cells may also suffer from extensive DNA rearrangement because in mature sperm cells, internalization of foreign DNA may activate certain endogenous nucleases in these cells. Maione, B. et. al. (1997) Activation of Endogenous Nucleases in Mature Sperm Cells upon Interaction with Exogenous DNA, DNA and Cell Biology, Vol. 16, pp. 1087-1097. Such rearrangement could threaten the usefulness of genetically modified animals using this technique.
Other work with sperm cells as vector have focused on the use of either lipid-based agents or electroporation to deliver foreign DNA into the sperm cells. Smith, supra; Rottman R., et. al. (1996) Liposome-mediated Gene Transfer via Sperm Cells. High Transfer Efficiency and Persistence of Transgenes by Use of Liposomes and Sperm Cells and a Murine Amplification Element, Journal of Animal Breeding and Genetics, Vol. 113, pp. 401-411; PCT Publications WO 99/42569, WO 99/40213, and WO 97/11597. Such methods may also suffer from the same problem of DNA internalization and exposure to nucleases that could cause rearrangement of the foreign DNA being introduced. In addition, lipid-based agents, which are often toxic, and electroporation may require extensive experimentation to prevent the death or the loss of sperm cell motility. Other techniques have also focused on using recombinant virus infection, as disclosed in PCT Publications WO 99/38991, or on using a “gene gun” with micro-carriers, as disclosed in PCT Publication WO 93/24626, to introduce foreign DNA into sperm cells. Such techniques may be technically challenging and may also affect the viability and motility of the sperm cells. They may also suffer from the same problem of DNA internalization and exposure to nucleases that could cause rearrangement of the foreign DNA being introduced.
Since 1989, researchers have reported the use of sperm cells as vectors in different animals ranging from insects, marine animals, amphibians, birds, and mammals. Smith, supra. However, few reported that the genetic modification was observed in viable mature offspring. Smith, supra. More problematic is the fact that some reports used only PCR analysis to verify the existence of the foreign DNA in the cells. These reports are summarized in table one of Gandolfi, F. (1998) Spermatozoa, DNA Binding and Transgenic Animals, Transgenic Research, Vol. 7, pp. 147-155. Since PCR cannot distinguish between foreign DNA transmitted through episomes or through the chromosomal DNA, Gandolfi has questioned the value of these reports stating that it “opens up an important argument relating to appropriate evaluation of the results described in some reports.” Gandolfi, supra. Episomal transmission is not as desirable as chromosomal transmission since the episome may be lost during subsequent cell division, and the desired effect of genetic modification may never be expressed in adult animals.
Because an easy, non-toxic, and efficient way of genetically modifying animals, especially in the field of transgenic chicken systems, can greatly advance transgenic technology, a new way of using sperm cells for delivering genes into animals is needed. The present invention provides these advantages among other benefits. Rather than manipulate a host oocycte by microinjection, the present invention incorporates use of agents which link exogenous DNA to a sperm cell, allowing entry of the foreign DNA into a host cell. Also included are systems for expressing therapeutic proteins in the transgenic embryos or animals created by the present methods.